Previously known devices of this type, for example, that in accordance with Swedish Patent 341,231, show a deaeration chamber provided with a diaphragm valve actuated by pump pressure and arranged to interconnect the deaeration chamber and pump intake. The deaeration chamber is, furthermore, provided with a float-control valve which opens and closes the vent opening of the deaeration chamber to the atmosphere. If the air content of the oil is extremely high, resulting in severe foaming and a reduction in the density of the oil in the deaeration chamber, the oil will be unable to lift the float sufficiently to close the vent opening. As a result, oil foam will escape uncontrollably from the vent opening, resulting in oil leakage.
Other known devices in which the vent is opened and closed by a float also exhibit the same disadvantages under conditions of severe foaming. One such example is the device disclosed in U.S. Pat. No. 4,579,664, in which the float must possess a certain weight to close the oil outlet from the deaeration chamber.